Steam rapidly reduces the swelling capacity of bentonite

Sodic bentonite is widely used for industrial and scientific purposes, especially as a sealing agent1, because of its great ability to swell in water2,3. It has been proposed for use in high-level nuclear waste repositories as an impermeable barrier surrounding the waste package, or to fill tunnels, shafts and rooms. In some repositories, such as those planned in basalt, bentonite would be expected to be subjected to temperatures possibly up to 300 °C (refs 4, 5). As the waste would be approximately 600–900 m below the water table in fractured rock, the repository is expected to fill first with steam and then with liquid water4,6. Reaction of bentonite with liquid water produces a minimal loss of swelling capacity, but I show here that reaction with water vapour at 150–250 °C results in rapid irreversible loss of most of the swelling capacity. This causes very large increases in permeability of sand–bentonite mixtures7, thereby reducing the ability of a bentonite barrier to retard the flow of groundwater in proposed high-level nuclear waste repositories.